What's causing my confusion is that rockstar put painstaking detail into some of the landmarks, and "par for the course" seems to be that if it's part of the map, you can probably bet that there's something identical or *very close" somewhere in the city.

They can be pretty hit or miss, though. Look at all the stuff missing in Venice. Most notable (IMO) are Abbot Kinney and Windward Circle. Meanwhile, MDR in the game has a helicopter tour area, which I don't recall one existing in real-life MDR.

They do do a good job at details but they also miss things or just create things. Like with the map the user replied to you with, there is a ton of real-world locations but then you have things like the junkyard area north/northwest of the Forum that has no real-world equivalent. For some places the locations do well at using real buildings while others don't. A good example is the homes in the Venice Canals versus the homes in Echo Park.